Selenium is recognized as a chemopreventive agent against various forms of cancer (e.g., colon, prostate, lung and liver), against heart disease, and as an antiviral agent. This element also likely plays a role in delaying the aging process, in delaying the progression of AIDS in HIV positive patients, in immune function, in mammalian development and in male reproduction. The beneficial effects of this element are due, at least in part, to its presence in selenoproteins as the amino acid, selenocysteine (Sec). Our program focuses on the means by which Sec is incorporated into protein and the role of specific selenoproteins in human health. As Sec tRNA is the only known tRNA to govern the expression of an entire class of proteins, the selenoproteins, we produced transgenic mice encoding mutant Sec tRNA transgenes, produced mice lacking the Sec tRNA gene (i.e., the standard Sec tRNA gene knockout mouse), produced mice carrying a conditional knockout of the Sec tRNA gene as model systems to modulate selenoprotein synthesis and using RNAi have knocked down several selenoproteins and generated a RNAi mouse, and therefore, assess the roles of these selenium-containing proteins in health. We generated transgenic mouse lines carrying mutant Sec tRNA transgenes that have either a T -> A base change in the wobble position of the anticodon or an A -> G base change at position 37. Both mutant tRNAs alter selenoprotein biosynthesis in a protein and tissue specific manner. We are now examining these mouse lines to respond to viral infection, to carcinogens and to diets with varying levels of selenium in a genetic background encoding a cancer driver gene. We have rescued standard Sec tRNA gene knockout mice that are embryonic lethal with transgenic mice carrying wild type Sec tRNA transgenes and are presently analyzing which selenoproteins are rescued. Using loxP-Cre technology, we have mated the conditional knockout mice with different Cre mice to selectively remove the Sec tRNA gene (and therefore alter selenoprotein expression) in various tissues including liver, prostate, brain and breast. In this way, we can assess the role of selenoproteins in liver, prostate, brain and breast malignancies. This past year we have totally knocked out selenoprotein expression in liver and have shown that selenoprotein expression is essential of normal liver function. In studies examining the role of specific selenoproteins in human health, we previously observed a correlation between several characteristics of the 15 kDa selenoprotein (Sep15) and cancer etiology. We generated a knockout mouse and are examining the consequences of removing this gene from the mouse genome. We completed an analysis of one selenoprotein in Drosophila using RNA interference (RNAi) to specifically remove it, have also knocked down a selenoprotein, SelH, in mammalian tissue using RNAi and made a RNAi transgenic mouse of SelH, and are analyzing the consequences of removing SelH expression in mice. Each of the above studies is directed at understanding the roles of selenium-containing proteins in health.